Color-developing sheet for pressure-sensitive recording sheets

ABSTRACT

A color-developing sheet for pressure-sensitive recording sheets, said sheet comprising as a color-developing agent at least one 2,2&#39;-bisphenolsulfone zinc salt selected from compounds of the general formula ##STR1## wherein R 1  and R 2  are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group.

BACKGROUND OF THE INVENTION

This invention relates to pressure-sensitive recording sheets, and more specifically, to a color-developing sheet for pressure-sensitive recording sheets which contains a novel color developing agent.

Pressure-sensitive recording sheets are also known as carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies simultaneously. The color is based on a color forming reaction between an electron-donating colorless dye and an electron-accepting color developer.

The structure of a pressure-sensitive recording sheet and the mechanism of color formation are illustrated generally with reference to the accompanying drawing. The back surface of each of a top (CB: Coated Back) sheet 1 and a middle (CFB: Coated Front and Back) sheet 2 is coated with microcapsules 4 having a diameter of several microns to ten and several microns and composed of a shell of a polymeric film such as gelatin and a solution of a colorless color-forming pressure-sensitive dye in an involatile oil enclosed therein. The surface of each of the middle (CFB) sheet 2 and the bottom (CF: Coated Front) sheet 3 is coated with a layer 5 containing a color developing agent having the property of reacting with the colorless dye upon contact therewith and thus producing a color. A sheet coated with a coating composition containing a color developer, such as the CFB sheet and the CF sheet, is called a color-developing sheet. When a localized pressure is applied by a writing instrument 6 (or a typewriter or the like) to a pressure-sensitive recording sheet composed of a multiply structure of the CB sheet 1, CFB sheet 2 and CF sheet 3 so that the microcapsule-coated surface faces the surface coated with the color developer-containing layer 5, the microcapsules 4 under the applied pressure break and the solution of the colorless dye moves to the color developer-containing layer 5. Thus, the dye reacts with the color developer to form a colored image 7 in the desired pattern of recording. In the pressure-sensitive recording sheet illustrated in the drawing, only one CFB sheet is interposed between the CB sheet 1 and the CF sheet 3, but if desired, two or more CFB sheets may be interposed.

Conventionally known electron-accepting color developing agents include (1) inorganic solid acids such as acid clay (Fuller's earth) or attapulgite, (2) substituted phenols and diphenols, (3) p-substituted phenol formaldehyde polymers, and (4) metal salts of aromatic carboxylic acids. These color-developing agents, however, are not entirely satisfactory. For example, the inorganic solid acids adsorb gases or moisture in the air and cause yellowing of the sheet or are deteriorated in color-forming property. With the substituted phenols and diphenols, the ultimate density of color is insufficient. The p-substituted phenol formaldehyde polymers (for example, p-phenyl phenol novolak resin) have superior color-forming properties, but have the defect that the coated sheet undergoes yellowing upon exposure to sunlight. The aromatic carboxylic acid metal salts are superior in color-forming ability, light fastness of the colored image and resistance to yellowing under light, but their water resistance and plasticizer resistance are not entirely satisfactory.

SUMMARY OF THE INVENTION

It is a first object of this invention to provide a color-developing sheet which exhibits an equivalent or higher color-developing ability to or than developing sheets containing conventional inorganic solid acids or p-phenylphenol novolak resin as a color developing agent, and which give colors having superior water resistance and plasticizer resistance.

A second object of this invention is to provide a color developing sheet which gives a colored image having superior light fastness and undergoing little decrease in density with time, and whose tendency to yellowing under sunlight or the like is drastically reduced.

A third object of this invention is to provide a color-developing sheet which lends itself to very advantageous handling and storage.

According to this invention, there is provided a color-developing sheet for pressure-sensitive recording sheets, said sheet comprising as a color-developing agent at least one 2,2'-bisphenolsulfone zinc salt selected from the group consisting of compounds of the formula ##STR2## wherein R₁ and R₂ are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group.

In a preferred embodiment, the present invention provides a color-developing sheet for pressure-sensitive recording sheets, said color-developing sheet comprising both the aforesaid 2,2'-bisphenolsulfone zinc salt and at least one polyvalent metal compound selected from the group consisting of oxides, hydroxides and carbonates of zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel, manganese and barium.

The 2,2'-bisphenolsulfone zinc salts of formula (I) are known compounds. However, it has not been known that these compounds form a color upon contact with a colorless pressure-sensitive dye to form a colored image having superior fastness characteristics, and are thus suitable as a color-developing agent for pressure-sensitive recording sheets. In addition, it has been difficult to anticipate the suitability of the 2,2'-bisphenolsulfone zinc salts as a color-developing agent.

Free bisphenolsulfone, a precursor for the zinc salts of formula (I), has been found to have little or no color-forming ability when used as a color-developing agent for pressure-sensitive recording sheets.

The color-developing sheet of this invention containing the zinc salt of formula (I) has an equivalent or greater color-forming ability to or than color-developing sheets containing inorganic solid acids or p-phenyl phenol novolak resin as a color-developing agent, and the formed image has good resistance to fading under the action of water, plasticizers, light, etc. The color-developing sheet of this invention also has much improved resistance to yellowing under exposure of sunlight or the like, and lends itself to very advantageous handling and storage.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The subject matter of this invention is a color-developing sheet for pressure-sensitive recording sheets, said color-developing sheet comprising as a color-developing agent at least one 2,2'-bisphenolsulfone zinc salt selected from the group consisting of compounds of the general formula ##STR3## wherein R₁ and R₂ are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group.

2,2'-bisphenolsulfone used to produce the zinc salt of formula (I) is produced, for example, by (i) oxidizing with hydrogen peroxide a 2,2'-bisphenol sulfide compound obtained by the reaction of a p-substituted phenol with sulfur dichloride, or (ii) oxidizing with hydrogen peroxide a 2,2'-bisphenol sulfoxide compound obtained by the reaction of a p-substituted phenol with thionyl chloride. Examples of the p-substituted phenol that can be used in the above reaction include p-halophenols such as p-chlorophenol and p-bromophenol; p-alkylphenols such as p-cresol, p-amylphenol, p-octylphenol, p-tertiary butylphenol and p-nonylphenol; p-cycloalkylphenols such as p-cyclohexylphenol; p-(α,α-dimethylbenzyl) phenol; and p-phenylphenol. Among these, p-octylphenol, p-nonylphenol, p-cyclohexylphenol, and p-(α,α-dimethylbenzyl)phenol are preferred. These p-substituted phenols may be used alone or in combimation with one another. When two or more p-substituted phenols are used, there can be obtained a mixture composed of two or more 2,2'-bisphenolsulfones having the formula ##STR4## in which two R groups are different from each other.

Some known methods can be applied to the production of the zinc salt of formula (I) from the resulting 2,2'-bisphenolsulfone or its mixture.

One method comprises reacting an alkali metal salt of the 2,2'-bisphenolsulfone with a water-soluble zinc salt in a solvent capable of dissolving both. According to this method, the 2,2'-bisphenolsulfone is first reacted with an alkali metal hydroxide or alkoxide to form the alkali metal salt of 2,2'-bisphenolsulfone or its solution in water, an alcohol or in a mixture of water and an alcohol. Then, the product is reacted with the water-soluble zinc salt.

Specifically, when the 2,2'-bisphenolsulfone zinc salt of formula (I) is to be produced, at least 2 moles of the alkali metal hydroxide or alkoxide and at least one mole of the water-soluble zinc salt are used per mole of the 2,2'-bisphenolsulfone. Examples of the water-soluble zinc salt that can be used are inorganic acid salts such as zinc chloride, zinc sulfate and zinc nitrate and organic acid salts such as zinc oxalate and zinc acetate.

The zinc salt of formula (I) can also be produced by reacting 2,2'-bisphenolsulfone with an organic acid salt as zinc oxalate, or zinc acetate in an organic solvent at an elevated temperature. Useful organic solvents for this reaction include methanol, ethanol, butanol, acetone, benzene, toluene, and methylene chloride. It is especially effective to dissolve the 2,2'-bisphenolsulfone compound in such a solvent.

The zinc salts of general formula (I) are used as color-developing agents either alone or in combination with each other according to the purpose of use.

The color-developing agent in accordance with this invention can be used jointly with known color-developing agents, such as inorganic solid acids (e.g. activated acid clay), organic polymers (e.g., phenolformaldehyde resin), and aromatic carboxylic acid metal salts, without any deleterious effect.

In one preferred embodiment of this invention, 0.01 to 10 parts by weight, preferably 0.2 to 5 parts by weight, per part by weight of the color-developing agent of at least one polyvalent metallic compound selected from the group consisting of oxides, hydroxides and carbonates of zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel and manganese is coated together with the color-developing agent of formula (I) on a sheet. Examples of the polyvalent metal compounds are zinc oxide, magnesium oxide, aluminum oxide, calcium oxide, lead oxide, titanium oxide, manganese oxide, magnesium hydroxide, aluminum hydroxide, calcium hydroxide, zinc hydroxide, and calcium carbonate. When these polyvalent metal compounds are used, the resulting color-developing sheet has improved resistance to yellowing under light, and the rate of color development becomes faster and the color density increases. Moreover, the light fastness, plasticizer resistance and storage stability of the formed image are improved at the same time. Among the aforesaid polyvalent metal compounds, zinc oxide is most practical.

The color developing sheet of this invention can be prepared by various methods. A typical method comprises coating a sheet such as paper with a water-base coating color containing the color-developing agent suspended therein. The coating color is conditioned to have a suitable viscosity and suitable coating characteristics by incorporating a kaolin clay, calcium carbonate, starch, etc. in a synthetic or natural latex, and desirabably contains 10 to 70% by weight of the color-developing agent based on the total solids content. When the content of the color-developing agent is less than 10% by weight, the color-developing agent cannot exhibit suffices color-forming properties. When it exceeds 70% by weight, the characteristics of the surface of the resulting color-developing sheet are deteriorated. The amount of the coating color to be coated on the base sheet is at least 0.5 g/m², preferably 1 to 10 g/m², as solids content. In other words, at least 0.05 g/m², preferably 0.1 to 7 g/m², of the color-developing agent is applied to the sheet.

The color-developing sheet of this invention can also be produced by an alternative method which comprises dissolving or suspending the color-developing agent in an organic solvent such as alcohols, hydrocarbons or ketones, optionally incorporating a kaolin clay, calcium carbonate, etc., and coating the resulting solution or suspension on a sheet such as paper. The amount of the coating solution or suspension is at least 0.05 g/m², preferably 0.1 to 7 g/m², as the color-developing agent.

Still another method for producing the color-developing sheet of this invention comprises incorporating the color-developing agent into a sheet-forming stock, and subjecting the mixture to a sheet-forming process.

The amount of the color-developing agent can be reduced in the present invention, and the concentration, viscosity, etc. of the coating composition can be varied over relatively wide ranges. Accordingly, both on-machine coating and off-machine coating are possible, and great advantages can be obtained not only in the properties of the color-developing sheet, but also in the process steps of sheet production.

The following Examples specifically illustrate the present invention.

SYNTHESIS EXAMPLE 1

In a mixture of carbon tetrachloride and n-hexane, p-tert-octyl phenol was reacted with sulfur dichloride to form 2,2'-bis(p-tert-octylphenol)sulfide. The sulfide was oxidized with hydrogen peroxide in glacial acetic acid to form 2,2'-bis(p-tert-octylphenol)sulfone. Recrystallization from carbon tetrachloride/n-hexane afforded a purified product having a melting point of 142° to 144° C.

Then, 9.48 g (0.02 mole) of the resulting 2,2'-bisphenolsulfone was added to a solution of 1.6 g (0.04 mole) of sodium hydroxide in 100 ml of ethyl alcohol. The solution was stirred for 1 hour under reflux, and then cooled to 30° C. A solution of 2.73 g (0.02 mole) of zinc chloride in 50 ml of ethyl alcohol was added, and the mixture was refluxed for 1 hour. The reaction mixture was poured into 500 ml of ice water. The precipitated white crystals were collected by filtration, and dried to afford 10.6 g of 2,2'-bis(p-tert-octylphenol)sulfone zinc salt (to be referred to as compound No. 1) corresponding to general formula (I).

SYNTHESIS EXAMPLE 2

Pale yellow 2,2'-bis(p-tert-butylphenol)sulfone was prepared in the same way as in Synthesis Example 1 using p-tert-butylphenol.

To a solution of 1.38 g (0.06 mole) of metallic sodium in 180 ml of ethyl alcohol was added 14.2 g (0.03 mole) of the aforesaid 2,2'-bisphenolsulfone. The mixture was stirred for 1 hour under reflux, and then cooled to 30° C. The reaction mixture was added to a solution of 4.09 g of zinc chloride in 100 ml of ethyl alcohol. The mixture was stirred for 4 hours at 30° C. The precipitated crystals were separated by filtration, and ethyl alcohol was distilled off under reduced pressure from the filtrate to afford 13.6 g of pale yellow crystals of 2,2'-bis(p-tert-butylphenol)sulfone zinc salt (to be referred to as compound No. 2) corresponding to general formula (I).

SYNTHESIS EXAMPLE 3

In dichloroethane, 2 moles of p-cyclohexylphenol was reacted with 1 mole of thionyl chloride to form 2,2'-bis(p-cyclohexylphenol)sulfoxide. The product was then oxidized with hydrogen peroxide in glacial acetic acid to form 2,2'-bis(p-cyclohexylphenol)sulfone. Recrystallization from carbon tetrachloride afforded white crystals having a melting point of 176° to 178° C. Using this product, 2,2'-bis(p-cyclohexylphenol)sulfone zinc salt (to be referred to as compound No. 3) corresponding to general formula (I) was prepared in the same way as in Synthesis Example 1.

SYNTHESIS EXAMPLE 4

Using p-phenylphenol, 2,2'-bis(p-phenylphenol) sulfone zinc salt (to be referred to as compound No. 4) was prepared in the same way as in Synthesis Example 1.

EXAMPLES 1 to 4

Using each of compounds Nos. 1 to 4 in Synthesis Examples 1 to 4 as a color-developing agent, a suspension of the following formulation was prepared by means of a sand grinding mill.

    ______________________________________                                                         Parts by weight                                                ______________________________________                                         Color-developing agent                                                                           24.5                                                         Sodium ligninsulfonate                                                                           3.0                                                          Water             42.5                                                         ______________________________________                                    

A coating composition of the following formulation was prepared by using the above suspension.

    ______________________________________                                                         Parts by weight                                                ______________________________________                                         Suspension        50                                                           Kaolin            100                                                          Styrene/butadiene latex                                                        (concentration 50%)                                                                              15                                                           Oxidized starch   15                                                           Water             180                                                          ______________________________________                                    

The coating composition was coated on a sheet of fine paper and dried so that the amount of the coating composition applied was 6 g/m² upon drying. Thus, a color-developing sheet was obtained.

COMPARATIVE EXAMPLE 1

A suspension and a coating composition were prepared in accordance with the same formulations as in Example 1 using 2,2'-bis(p-tert-octylphenol)sulfone which is a precursor of the 2,2'-bis(p-tert-octylphenol)sulfone zinc salt (compound No. 1) obtained in Synthesis Example 1. The coating composition was coated on a sheet of fine paper and dried so that the amount of the coating composition was 6 g/m² upon drying. Thus, a color-developing sheet was obtained.

COMPARATIVE EXAMPLE 2

A color-developing sheet was prepared in the same way as in Comparative Example 1 using 2,2'-bis[(p-(α,α-dimethylbenzyl)phenol]sulfone.

COMPARATIVE EXAMPLE 3

A glass reactor was charged with 170 g of p-phenylphenol, 22.5 g of 80% para-formaldehyde, 2.0 g of p-toluenesulfonic acid and 200 g of benzene, and with stirring, the contents were heated. While water generated as a result of the reaction was distilled off out of the reactor as an azeotrope with benzene, they were reacted at 70° to 80° C. for 2 hours. Then, 320 g of a 10% aqueous solution of sodium hydroxide was added, and benzene was distilled off by steam distillation. Dilute sulfuric acid was added dropwise, and the precipitated p-phenylphenol/formaldehyde polymer was collected by filtration, washed with water, and dried to afford 176 g of a white powder. A color-developing sheet was produced in the same way as in Comparative Example 1 using the resulting p-phenylphenol/formaldehyde polymer.

COMPARATIVE EXAMPLE 4

A color-developing sheet was prepared in the same way as in Comparative Example 1 except using zinc 3-[4'-(α,α-dimethylbenzyl)phenyl]-5-(α,α-dimethylbenzyl)salicylate.

TEST EXAMPLES

The properties of the color-developing sheets produced in Examples 1 to 4 and Comparative Examples 1 to 4 were tested by the following methods. The results are tabulated below.

(1) Color-developing rates of the color-developing sheet

A commercially available CB sheet (NW-50T, a product of Jujo Paper Co., Ltd.) containing crystal violet lactone as a main pressure-sensitive dye and a sample color-developing sheet are laid so that the coated surfaces of the sheets contacted each other. A pressure is applied to the multiply sheet by an electric typewriter to form a cobalt blue color. The reflectance I_(o) of the sheet before color formation, the reflectance I₁ of the sheet two minutes after color development, and the reflectance I₂ of the sheet 24 hours after color development are measured by using a Hunter Reflectmeter (using an amber filter). The initial color-developing rate (J₁) and the ultimate color-developing rate (J₂) are calculated in accordance with the following equations. ##EQU1##

Higher initial and ultimate color-developing rates are preferred.

(2) Retention of color intensity (Light-fade resistance)

The color-developing sheet having a color formed by the method described in (1) is exposed to a fade-o-meter for 2 hours and 6 hours, respectively. The color-developing rates of the sheets are calculated in the same way as in (1) above as J₃ and J₄. The retention of color is calculated in accordance with the following equations. ##EQU2##

Higher color retention values are preferred.

(3) Retention of whiteness (Resistance to yellowing)

The color-developing sheet before color formation is exposed to a fade-o-meter for 6 hours. The reflectance K₁ of the sheet before the exposure, and its reflectance K₂ after the exposure are measured by using a Hunter Reflectmeter (using a blue filter). The retention of whiteness (H) is calculated in accordance with the following equation. ##EQU3##

Higher whiteness retention values show less yellowing of the sheet under light.

(4) Plasticizer resistance of a colored image

A small amount of dioctyl phthalate (a plasticier for vinyl chloride resins) is coated on the colored surface of the color-developing sheet after color formation by the method described in (1) above. A change in density of the color was observed with unaided eyes immediately after coating and two weeks after coating.

(5) Water resistance of colored image

The color-developing sheet after color formation by the method described in (1) is dipped in water. A change in the density of the color was observed with unaided eyes two 1 hours after dipping.

    __________________________________________________________________________                                       Plasticizer resis-                           Color                             tance of colored                                                                         Water                              developing agent              White-                                                                             image     resistance                                    Amount                                                                              Color         ness                                                                               Immedia-  of colored                         Color-de-  (parts                                                                              developing                                                                             Retention of                                                                         reten-                                                                             tely 2 weeks                                                                             image after                        veloping                                                                            Compound                                                                             by   rates (%)                                                                              color (%)                                                                            tion                                                                               after                                                                               after                                                                               2 hour                             Sheet*                                                                              No.   weight)                                                                             J.sub.1                                                                             J.sub.2                                                                           2 hrs                                                                             6 hrs                                                                             (%) coating                                                                             coating                                                                             dipping                            __________________________________________________________________________     Ex. 1                                                                               1     24.5 40.9 48.0                                                                              52.1                                                                              15.8                                                                              90.7                                                                               No   Slight                                                                              No decrease                                                          decrease                                                                            decrease                                                                            in color                                                             in color                                                                            in color                                                                            density                                                              density                                                                             density                                                                   No   Slight                                  Ex. 2                                                                               2     24.5 30.6 46.4                                                                              52.3                                                                              16.1                                                                              90.3                                                                               decrease                                                                            decrease                                                                            No decrease                                                          in color                                                                            in color                                                                            in color                                                             density                                                                             density                                                                             density                                                              No   Slight                                  Ex. 3                                                                               3     24.5 39.0 43.4                                                                              52.0                                                                              15.2                                                                              86.5                                                                               decrease                                                                            decrease                                                                            No decrease                                                          in color                                                                            in color                                                                            in color                                                             density                                                                             density                                                                             density                                                              No   Slight                                  Ex. 4                                                                               4     24.5 35.0 43.5                                                                              52.3                                                                              15.5                                                                              86.5                                                                               decrease                                                                            decrease                                                                            No decrease                                                          in color                                                                            in color                                                                            in color                                                             density                                                                             density                                                                             density                            CEx. 1                                                                              Free  24.5 no color                                                                               -- -- --  --   --   --                                      sulfone    formed                                                         CEx. 2                                                                              Free  24.5 no color                                                                               -- -- --  --   --   --                                      sulfone    formed                                                         Cex. 3                                                                              Known 24.5 41.0 47.0                                                                              50.7                                                                              10.7                                                                              82.0                                                                               Marked                                                                              Marked                                                                              No decrease                             color                        decrease                                                                            decrease                                                                            in color                                develop-                     in color                                                                            in color                                                                            density                                 ing agent                    density                                                                             density                                 CEx. 4                                                                              Known 24.5 23.5 46.2                                                                              68.3                                                                              13.4                                                                              90.0                                                                               Slight                                                                              Marked                                                                              No decrease                             color                        decrease                                                                            decrease                                                                            in color                                develop-                     in color                                                                            in color                                                                            density                                 ing agent                    density                                                                             density                                 __________________________________________________________________________      *Ex. = Example;                                                                CEx. = Comparative Example                                               

It can be seen from the table that the color-developing sheets of this invention have a marked improvement in color-forming properties than the color-developing sheets of Comparative Examples 1 and 2 (prepared in the same way as in Example 1 using free sulfones which are precursors of the color-developing agents of the present invention). In other words, while the free sulfones have no color-forming properties, the zinc salts thereof have very good color-forming properties. Furthermore, the color-developing sheets of this invention have equivalent or slightly better color-forming properties (the color-developing rate) and water resistance to or than the color-developing sheets of Comparative Examples 3 and 4 (color-developing sheets containing known color developing-agents), but have much better light fading resistance (color retention), resistance to yellowing under light (whiteness retention) and plasticizer resistance than the latter.

Since the color produced by the color-developing sheet of this invention is stable to light and plasticizers or the like and scarcely undergoes a decrease in density, it can be used in applications for which conventional color-developing sheets are unsuitable because of the need for long-term storage stability. Hence, the color-developing sheet of this invention has a very great practical significance. 

What we claim is:
 1. A color-developing sheet for pressure-sensitive recording sheets, said color-developing sheet comprising a base sheet and associated therewith a color-developing agent comprising at least one 2,2'-bisphenolsulfone zinc salt selected from the group consisting of compounds of the following formula ##STR5## wherein R₁ and R₂ are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group.
 2. The color-developing sheet of claim 1 wherein the 2,2'-bisphenolsulfone zinc salt is a product from at least one p-substituted phenol selected from the group consisting of p-halophenols, p-alkylphenols, p-cycloalkylphenols, p-(α,α-dimethylbenzyl)phenol and p-phenylphenol.
 3. The color-developing sheet of claim 1 which further comprises at least one polyvalent metal compound selected from the group consisting of oxides, hydroxides and carbonates of zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel and manganese.
 4. The color-developing sheet of claim 3, wherein the polyvalent metal compound is zinc oxide.
 5. The color-developing sheet of claim 3 or 4, wherein the amount of the polyvalent metal compound is from 0.01 to 10 parts by weight, per part by weight of the color-developing agent.
 6. The color-developing sheet of claim 5, wherein the amount of the polyvalent metal compound is from 0.2 to 5 parts by weight, per part by weight of the color-developing agent.
 7. The color-developing sheet of claim 1, wherein the color-developing sheet comprises the base sheet and a layer of a coating composition containing said color-developing agent coated on said base sheet.
 8. The color-developing sheet of claim 7, wherein the color-developing agent is applied to the base sheet in an amount of from 0.1 to 7 g/m².
 9. The color-developing sheet of claim 8 wherein said coating further comprises from about 0.01 to 10 parts by weight of zinc oxide per part by weight of the color-developing agent.
 10. The color-developing sheet of claim 1, wherein the color-developing agent is incorporated in said base sheet.
 11. The color-developing sheet of claim 1, wherein the zinc salt of formula (I) is selected from the group consisting of 2,2'-bis(p-tert-octylphenol)sulfone zinc salt, 2,2'-bis(p-tert-butylphenol)sulfone zinc salt, 2,2'-bis(p-cyclohexylphenol)sulfone zinc salt, and 2,2'-bis(p-phenylphenol)sulfone zinc salt. 